Batteries are used in a wide range of electrical systems, e.g., uninterruptible power supply (UPS) systems. Batteries can be used to provide backup power, which discharges the batteries, and may be charged to replenish depleted energy used to power electronic equipment.
The terminal voltage of batteries during discharge may be used to recognize problems and assess and predict battery capacity. Small voltage changes at the battery terminals can reveal information such as the battery internal resistance and the remaining capacity of the battery if combined with other information such as the elapsed time since the beginning of discharge and level of discharge current.
The voltage can be measured and converted from analog (continuous) to digital (samples) and analyzed by a processor to diagnose/predict problems. Analog-to-digital converters (ADCs) have inherent amounts of undesirable noise created by the sampling process that affects the accuracy and usefulness to a computer of the signals output by ADCs. The noise causes the digital outputs to change sample to sample in both positive and negative directions. This occurs even though an actual battery terminal analog voltage of interest remains the same or changes only in a negative direction during a discharge. If the rendered terminal voltage value contains noise that is in excess of the resolution that is used to assess battery performance, then the rendered value is unreliable for assessing battery performance.
A factor that influences battery operation is ambient temperature. A change in ambient temperature near a battery results, after a time delay, in a change in the battery's internal temperature. The time delay may be thought of as a thermal propagation delay; i.e., the time it takes for changes in the ambient temperature to slowly propagate through the battery case (thermal resistance) and into the internal part of the battery. Assuming a step increase in ambient temperature, the thermal delay forms an S-curve of internal battery temperature versus time where internal battery temperature takes time (typically several minutes) to reach the ambient temperature. The S-curve results from the internal temperature being constant, then increasing due to the increase in ambient temperature, first slowly and then more rapidly, and then leveling off, asymptotically approaching a steady state temperature. Because the battery case provides a barrier to the ambient heat from reaching the internal part of the battery, the slope of the curve is initially small and increases with time, forming an S-shaped curve (see FIG. 4).
Internal battery temperature affects operational characteristics of the battery. As battery temperature increases, the lead-acid battery voltage will increase and a proportionately lower charger float voltage will maintain the same charge current. Conversely, as battery temperature decreases, a higher charger float voltage is used to maintain a constant charge current. Increasing float voltage can warm the battery's internal temperature. If the input charge energy exceeds the battery's ability to convert the energy to chemical energy or to dissipate the excess energy, thermal runaway may occur. If the conditions are not corrected, the current may continue to increase until it is removed or the circuit opens, e.g., due to a rupture or explosion.
A battery, as used in this document, may comprise multiple batteries. The batteries may be connected in series to provide a cumulative voltage, and the series of batteries may be collectively considered as a single battery for many purposes.
There may be confusion, however, when maintenance personnel attempt to correlate actual physical location of individual batteries within a string of batteries with indications from associated monitoring equipment. The individual batteries may or may not be labeled with identifiers such as #1, #2, #3, yet the monitoring equipment adopts some means of relating an alarm, message or data to a specific battery. Hundreds of individual polarized units are often connected in series. The string terminal voltage is the sum of all the individual terminal voltages. Each of the individual units has a negative and positive terminal and these terminals are connected together in a negative to positive fashion to result in a string positive output at one end and a string negative output at the extreme opposite end. Monitoring equipment is connected to the individual battery terminals to enable measurement and testing at the individual battery level. The individual batteries are frequently assigned number labels for the purpose of identification by maintenance and service personnel. The monitoring equipment provides numbered connection wires that are correlated to the physical identification of the individual batteries.
There is, however, often no certainty that identification numbers assigned to the batteries may not be assigned with regard to polarity or reference point. For example, the battery labeled as #1 might start at either end of the string, irrespective of the string's output terminal polarity. The monitoring equipment, though, is polarity sensitive and could be damaged as a result of sensitive electronic components receiving unintended reverse currents from the batteries. Further, if polarity is disregarded, the monitoring equipment may incorrectly associate measured values with the batteries, resulting in maintenance being performed on the wrong batteries and the defective batteries detected by the monitoring equipment remaining in place.